Method and apparatus for in situ depositing of neutral Cs under ultra-high vacuum to analytical ends

ABSTRACT

The present invention relates to a method for modifying the electronic properties of a surface to analytical ends, such as SIMS or electron spectroscopy, characterised in that it comprises in situ deposition of pure neutral cesium (Cs 0 ), under ultra-high vacuum, said neutral cesium being enabled in the form of a collimated adjustable stream. The invention relates also to the special column designed for implementing the method and to the corresponding energy and/or mass analyser instrument.

FIELD OF THE INVENTION

The present invention relates to a new Secondary Ion Mass Spectroscopy(SIMS) method operating in the MCs_(x) ⁺ (x=1,2) mode. The method hasbeen carried out in a Cation Mass Spectrometer (CMS) developed andmodified by the inventors for reaching high secondary emission usefulyields combined with excellent depth and lateral resolution.

The invention also relates to the modified CMS, in particular by meansof a cesium deposition column which is coupled with traditional primarybombardment column.

PRIOR ART AND RELATED TECHNICAL BACKGROUND

Owing in particular to its excellent sensitivity and its good depthresolution, Secondary Ion Mass Spectrometry (SIMS) constitutes anextremely powerful technique for analysis of surfaces and thin films.Its main fields of application lie in the semiconductor, glass, organicand metallurgical composite materials.

SIMS instruments also allow the recording of ion images of the surfaceof the analysed sample. In this case, a thin ion probe sweeps thesurfaces of the sample and the secondary ions which have suitablyselected mass are recorded with respect to the position on the surfacefrom which they originate. An emerging field of application for thisimaging technique providing good lateral resolution combined withexcellent sensitivity is situated in particular in biology.

Alongside all its advantages, however, the SIMS technique suffers fromone major drawback: the measurements can only be quantified withdifficulty. The intensity of the measured signals is generally greatlydependent on the sample analysed, given that the ionisation yield of agiven sputtered element may vary by several orders of magnitudedepending on the composition of the matrix in which it is located. Thisphenomenon is known as the matrix effect.

In order to get round these problems linked to the matrix effect, theSIMS analyses are increasingly carried out in MCs_(x) ⁺ mode. Thismethod consists in incorporating cesium (Cs) in the material of interestand detecting positive ion clusters formed by the recombination of anatom of the element M in which one is interested together with one ortwo atoms of Cs. Given that these MCs_(x) ⁺ clusters are formed byatomic recombinations above the surface of the sample, the compositionof the matrix does not come any longer into play directly, consequentlyeliminating the matrix effect.

SIMS instruments of prior art have exclusively used a beam made of Cs⁺primary ions in order to perform analyses in the MCs_(x) ⁺ mode. Anumber of drawbacks are still to be listed with this respect.

Optimisation of the Cs Concentration to form MCs_(x) ⁺ Clusters

When the analyses in MCs_(x) ⁺ mode are carried out by bombarding thesample with a beam of Cs⁺ ions, this beam serves both for theincorporation of Cs in the material and for the sputtering of thesurface. In this case, the Cs concentration, which is a crucialparameter determining the sensitivity of the analysis, as shown in FIG.1, is set by the primary bombardment conditions—mainly the angle andenergy of impact—which can be adapted only in a very limited way onconventional SIMS equipment. Consequently, the Cs concentration ispractically fixed for a given type of sample and cannot be chosenfreely. As it is unlikely that the Cs concentration thus obtained willcoincide with the optimum concentration for the material in question,the analysis is not optimised.

Coupling of the Cs Concentration and the Depth Resolution

A second major disadvantage of the use of Cs⁺ ion bombardment relates tothe impossibility of separately choosing the Cs concentration (c_(Cs))implanted in the sample and the energetic and angular parameters of theprimary beam, given that the latter determine the value of c_(Cs). Nowthe primary bombardment conditions also considerably affect majoranalytical characteristics such as the depth resolution.

Pre-Equilibrium State

The introduction of Cs into the material by ion bombardment does notallow an optimum Cs concentration to be attained right from the firstatomic layer, given that the Cs atoms are implanted under the surface,as shown in FIG. 2, at greater or lesser depth depending on their impactenergy. Consequently, the analysis is inconclusive in thepre-equilibrium state which precedes the achieving of a constantconcentration of Cs (in a Cs⁺ bombardment) or Cs and Ga (in a Cs⁺ andGa⁺ bombardment).

Optimisation of Formation of Negative Secondary Ions

Furthermore, bombardments by electropositive elements are often used toraise the negative ion yield by several orders of magnitude. In thiscontext, the emission of negative secondary ions is greatly enhanced bythe presence of Cs atoms on the surface of the sample which isbombarded.

Aims of the Invention

The present invention aims at providing a new Secondary Ion MassSpectroscopy (SIMS) method operating in the MCs_(x) ⁺ (x=1,2) modepermitting to separately choose the Cs concentration implanted in thesample and the sputtering of the sample surface, leading thus tosimultaneous optimisation of the Cs concentration and analyticalparameters, such as depth resolution, which depend now exclusively onprimary bombardment conditions.

Particularly, the invention aims at permitting one, by depositingneutral Cs atoms on the sample surface, to vary the Cs concentrationcontinuously in the range quasi 0 to 100% to an optimum value in orderto maximise detected MCs_(x) ⁺ and Cs_(x) ⁺ signals for any kind ofsample.

Additionally, the invention aims at enabling an optimised signal to bemeasured right from the first atomic layer.

Another goal of the present invention is to provide aspecially-developed cesium column with significant service life increaseand designed to considerably reduce the risk of contaminating of thecesium deposit as well as of the analysis chamber with traces of otherelements.

SUMMARY OF THE INVENTION

A first object of the present invention relates to a method formodifying the electronic properties of a surface to analytical ends,characterised in that it comprises in situ deposition of neutral cesium(Cs⁰), under ultra-high vacuum (residual pression of about 10⁻⁹–10⁻¹⁰mbar), said neutral cesium being enabled in the form of a collimatedadjustable stream.

According to the invention, the stream of Cs⁰ is provided and collimatedin a column by means of:

-   -   a temperature adjustment of an evaporator comprising a metallic        cesium reservoir, and/or    -   an aperture control of a motorised obturator located in the path        of the cesium stream.

It was particularly contemplated by the inventors that said Cs⁰deposition be simultaneously accompanied by a primary bombardmentcomprising electrons and/or ions or neutral atoms or groups of atoms, orby an X-ray irradiation, intended to induce an emission of a beam ofparticles for analysis, out of the surface.

Preferably, the method of the invention is coupled to static or dynamicSecondary Ion Mass Spectroscopy (SIMS), preferably operating in theMCs_(x) ⁺ mode (x=1, 2).

Advantageously, the deposition rate of Cs⁰ is continuously adjustable inthe range from 0 to 10 Å/s, corresponding about to 0–4 monolayers persecond.

According to another preferred embodiment, the method of the inventionis coupled to electron spectroscopy, preferably Auger ElectronSpectroscopy (AES), Electron Energy Loss Spectroscopy (EELS), X-RayPhotoemission Spectroscopy (XPS) or Ultraviolet PhotoemissionSpectroscopy (UPS).

According to the SIMS embodiment, the secondary beam for analysiscomprises secondary electrons and/or Cs_(x) ^(n+) and/or MCs_(x) ^(n+)positive clusters and/or M^(n−) negative ions and/or M^(m+) positiveions, M being a constituent of the sample material made of an atom or agroup of atoms (n, m integers).

Advantageously, the sputtering and Cs introduction phases are decoupledduring analyses in the MCs_(x) ⁺ mode, in a simultaneous optimisation ofdeposited Cs concentration and analytical characteristics, such as thedepth resolution.

Still more advantageously, the depth resolution solely depends on thebombardment conditions for the analysis.

The method of the invention further enables a stream of a chemicalelement other than Cs, evaporated under ultra-high vacuum, to createsecondary emission for analytical purposes of M₁M₂ ^(n+) clusters or M₂^(m−) ions or M₂ ^(m+) ions (n, m integers) or electrons, wherein M₁ andM₂ are respectively the atoms or groups of atoms constituted by thechemical element other than Cs and the atoms or groups of atoms from thesample.

Still advantageous, the sole adjustable deposition rate of Cs⁰ or achemical element other than Cs to an optimised value enables to optimisethe intensity of the secondary particles emitted by the sample.

According to a preferred embodiment, the reservoir temperature range ismaintained between 70 and 90° C., corresponding to a pressure range from1.10⁻⁴ to 4.10⁻⁴ mbar and in that the stability of the deposition rateis about 2% over 60 minutes.

Under bombardment analysis mode, the deposited Cs⁰ concentration issolely related to the respective Cs and sample densities (ρ_(Cs),ρ_(M)), to the sputtering yield in the given bombardment conditions (Y)and to the ratio between the Cs⁰ erosion (V_(er)) and the deposition(V_(D)) rate (τ=V_(er)/V_(D))

A significant advantage of the invention resides in the fact that theuseful yield, i.e. the sensitivity, of the secondary emission species,preferably M^(n−), M^(m+), and still more preferably Cs_(x) ^(n+) andMCs_(x) ^(n+), is approximately solely related to said ratio (τ) and notto the respective erosion and deposition rate taken individually and inthat the secondary signal is optimisable by adjusting the Cs⁰ depositionrate to attain an optimum value of said ratio (τ).

Preferably, the stream of Cs₀ is automatically and continuously adaptedvia the obturator.

A second object of the invention relates to an energy and/or massanalyser instrument, for carrying out the method described, comprising aneutral cesium (Cs⁰) deposition column capable of delivering anadjustable and stable stream of pure neutral cesium, said column beingpreferably usable simultaneously with a primary bombardment or a primaryirradiation column.

The instrument preferably is a static or dynamic secondary ion massspectrometry (SIMS) instrument, comprising a primary bombardment columnand a secondary column equipped with secondary ion extraction means, amass spectrometer, preferably of the type TOF (Time-Of-Flight),quadrupolar or with magnetic sector and ion detection means.

A third object of the invention relates to a neutral cesium columnusable in an instrument, such as described. The neutral cesium columncomprises an evaporation block including a reservoir filled with puremetallic cesium, equipped with temperature control means, prolonged by atube up to a gun end piece located close to the sample and equipped withbeam collimation means.

Advantageously, said beam collimation means comprise a motorisedcontinuously adjustable obturator, preferably comprising a rotary diskusing a slit of continuously variable width, said disk being driven by astepper motor.

Still advantageously, at the operation temperature, the neutral cesium(Cs⁰) is in liquid state and the evaporation block lies with aninclination angle such as said liquid remains in the bottom of thereservoir under gravity effect.

Still advantageously, said tube and gun end piece equipped with beamcollimation means are further equipped with temperature control meansfor preventing condensation and obturation risks.

Preferably, the evaporator bloc is located in an external part which canbe isolated from the main chamber of the instrument by means of a gatevalve and capable of being separately pumped and vented.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the evolution of useful yields (i.e. sensitivity) as afunction of the Cs concentration for the aluminium sample (Cs⁺ orCs⁺/Ga⁺ primary ion bombardment).

FIG. 2 represents the depth evolution of Cs concentration implanted in asilicon sample for two different modes of analysis : pure Cs⁺bombardment and Cs⁺/Ga⁺ co-bombardment.

FIG. 3 represents the principle of analyses grouping together thesputtering and Cs incorporation stages (a) and those separating thesetwo stages (b).

FIG. 4 comparatively represents the depth evolution of the Csconcentration in silicon for three different modes of analysis : pureCs⁺ bombardment, Cs⁺/Ga⁺ co-bombardment and Ga⁺ bombardment with Cs⁰deposition, the latter corresponding to the difference with FIG. 2.

FIG. 5 represents a schematic overall view of the Cs⁰ evaporator of theinvention, wherein the external and internal parts are separated at thegate valve.

FIG. 6 represent a schematic representation of a preferred obturatorembodiment.

FIG. 7 represents the change in the Cs⁰ deposition rate on the sample asa function of the heating power transmitted to the neutral cesiumreservoir.

FIG. 8 represents the change in the Cs⁰ deposition rate onto the sampleas a function of the temperature (in mV) of said reservoir.

FIG. 9 represents the fluctuation of the Cs⁰ deposition rate indicatedby the quartz microbalance controller over a one-hour period for areservoir heating power of 85 W. The solid line indicates the averagevalue.

FIG. 10 represents the profile of the Cs⁰ beam recorded by determiningthe deposition rate for different positions of the quartz balance. Theabscissa 0 corresponds to the position located directly below thesecondary ion extraction nose. The continuous curve approximates theexperimental points by a Gaussian.

FIG. 11 represents the variation of the Cs concentration determinedexperimentally for four different Cs⁰ deposition rates with respect todifferent erosion rates for an aluminium sample.

FIG. 12 represents the experimental change (squares) and theoreticalchange (solid curve) of the Cs concentration as a function of theparameter τ for the aluminium sample.

FIG. 13 represents the change in the useful yield of the sputtered AlCs₂⁺ cluster of a sample of aluminium as a function of the characteristicparameter τ.

FIG. 14 represents the SIMS depth profile of a sample of Si subjected toan implantation of Mg and In.

FIG. 15 represents the SIMS depth profile of a Al sample subjected to animplantation of Ti and Cu.

FIG. 16 represents the SIMS depth profile of a sample of InP subjectedto an implantation of F and Al.

FIG. 17 respectively represents AlCs⁺, CuCs⁺, Cs⁺ and Cs₂ ⁺ secondaryion images of the same area of an Al/Cu grid.

DESCRIPTION AND ADVANTAGES OF THE INVENTION

The Laboratory for the Analysis of Materials (LAM) of the Centre deRecherche Public—Gabriel Lippmann has developed and installed on theCation Mass Spectrometer (CMS), which is a prototype scientificinstrument [1,2], a column which allows an adjustable and collimatedstream of neutral cesium (Cs⁰) to be deposited on the surface of thematerial sample to be analysed.

Using this new column, it was possible to introduce an analysistechnique consisting of a x^(y+) ion bombardment accompanied by adeposit of Cs⁰ at the surface of the sample. This experimental techniquepermits to avoid the constraints imposed by a Cs⁺ ion bombardment whichwere described above in the prior art section.

This new analysis technique provides an additional degree of freedom byseparating the sputtering and Cs introduction phases of analyses inMCs_(x) ⁺ mode with aiming at a simultaneous optimisation of the Csconcentration and of the major analytic characteristics such as thedepth resolution which mainly depend on the primary bombardmentconditions. The principle of the invention is roughly illustrated inFIG. 3.

When the optimum quantity of Cs is deposited in the form of neutralatoms on the surface of the sample, no sputtering process or atomicmixing of the target takes place and the depth resolution of theanalysis depends, advantageously, solely on the characteristicconditions of the bombardment produced by the sputtering/analysis gun.

On the other hand, the use of the Cs evaporator which deposits the Csatoms right from the surface of the sample enables an equilibrium stateto be attained from the first atomic layer, as shown in FIG. 4. This newanalysis technique therefore offers a considerable advantage in theanalysis of samples in which the interesting zone is in the closevicinity of the surface.

Finally, this same Cs⁰ column also enables optimisation of the negativesecondary ions by depositing the optimum quantity of Cs.

REFERENCES

-   [1] T. Mootz, B. Rasser, P. Sudraud, E. Niehuis, T. Wirtz, W.    Bieck, H. -N. Migeon, in A. Benninghoven, P. Bertrand, H. -N.    Migeon, H. W. Werner (Eds.), Secondary Ion Mass Spectrometry SIMS    XII, Elsevier, Amsterdam, 2000, p. 233–236.-   [2] T. Wirtz, B. Duez, H. -N. Migeon, H. Scherrer, Int. J. Mass    Spectrom. 209 (2001) 57.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

1. The Cs⁰ Evaporator

1.1. Operating Principle

To deposit alkaline metals, laboratories almost exclusively use getters,marketed by the Italian group SAES, containing the alkaline metal X inthe form of chromate X₂CrO₄ and a Zr_(x)Al_(y) type powder acting as areducing agent in order to release the X (i.e. Cs) atoms. When thismixture is heated by circulating an electric current, the reductionreaction is activated and the quantity of alkaline atoms thus produceddepends on the heating power. In the case of Cs, this reduction reactionis written as follows:2 Cs₂CrO₄+2.5 Zr→Cr₂O₃+2.5 ZrO₂+4 Cs

The use of such a getter on the CMS machine to carry out theoptimisations cited here above nevertheless presents several majordrawbacks. First of all, preliminary calculations have shown that, instandard bombardment conditions, a Cs deposition rate of approximately 1Å/s would be appropriate. To attain such a stream of Cs atoms on thesurface of the sample while at the same time choosing an acceptablesource-sample distance, it is necessary for the getter to emit at a rateof approximately 4.10¹⁴ atomes/s, which corresponds to 8.8.10⁻⁸ g/s. Nowas a 1 cm-long getter is filled with 4.4 mg of Cs and such a length maynot be exceeded in order to guarantee that the majority of the Cs atomsare deposited in the zone to be analysed, the Cs source would have aservice life of only 5.10⁴ s or 14 hours. As the stream of Cs atoms isnot collimated and therefore covers a much bigger space than necessary,the service life calculated in this way represents a theoretical upperlimit.

The impossibility of directing the Cs stream exclusively onto theanalysis zone is a second major disadvantage of getters. In addition tothe considerable loss of useful Cs atoms, such an uncollimated emissionis also very likely to contaminate the whole analysis chamber bydepositing a conductive and highly reactive film on all the surfaces,including ceramics serving as electrical insulators which will therebybe rendered ineffective.

Finally, the vapours emitted by commercial getters always contain tracesof other elements originating mainly in the reducing agents used. Thiscontamination of the Cs deposit is likely to increase the detectionlimits for certain elements given that the signal of the element inquestion would be affected by a background noise of varying degrees ofintensity.

In view of these service life drawbacks, contamination of the analysischamber and cleanliness of the Cs deposit, we decided not to use aconventional getter to equip the CMS machine of the invention.

As an alternative, we developed a source emitting a stream of Cs fromthe evaporation of pure metallic Cs. This configuration was intended toenable the elimination of at least the first two major disadvantages ofconventional getters mentioned above. Indeed, the service life can beincreased enormously (by a factor of 1000) given that it is possible tofill the reservoir with a large quantity of Cs (several grams) and giventhat the Cs beam can be collimated on the useful zone by using a gun endpiece with a small opening positioned close to the sample. Finally, asthe evaporator is loaded with pure metallic Cs, the risk ofcontaminating the Cs deposit with traces of other elements is alsoreduced.

1.2. Description

The Cs evaporator specially designed for the CMS machine of theinvention comprises two separate parts, as shown in FIG. 5. While thefirst part is located completely inside the main chamber of the CMSinstrument, the second part is mounted outside the chamber using a 40CFflange. Because of their respective positions with respect to the mainchamber, the two parts will henceforth be referred to as the “internal”and “external” parts.

1.2.1. External Part

The external part of the evaporator comprises an actual evaporationblock 1 and a tube (primary tube 2) guiding the stream of gaseous Cstowards the internal part of the evaporator. It can be isolated from themain chamber 3 of the CMS machine by means of a gate valve 4 and beindependently pumped or vented.

The evaporation block 1 is a solid cast stainless steel part. This blockcontains a cylindrical housing in which the reservoir 5 containing themetallic Cs slides. The maximum capacity of this reservoir is 7.6 g ofCs. To prevent the liquid Cs from flowing out into the whole volume ofthe evaporator, the evaporation block is mounted on the external part ofthe evaporator with an angle of inclination such that all the liquid Csremains at the bottom of the reservoir under the effect of gravity. Toobtain gaseous Cs, the housing of the reservoir is wound round with a“Thermocoax”-type heating wire 6 capable of supplying the thermal energynecessary for the evaporation of the Cs. The temperature of thereservoir is measured by means of a chromel-alumel thermocouple 7screwed to the end of the reservoir.

At the evaporation block exit, the Cs vapour escapes through an 8 mmhole in a stainless steel tube which also has a diameter of 8 mm and alength of 180 mm, which guides the gas towards the entry of the internalpart of the evaporator. To avoid condensation of the Cs on the walls ofthis guide tube, the tube is wound round with a heating wire 8 lodged ingrooves.

The external and internal parts are separated by a gate valve 4 enablingthe evaporation block to be isolated from the main chamber 3 when theevaporator 5 is not in operation. When the evaporator is in operation,the two parts are brought back into contact with each other by movingthe external part by means of a bellows-operated translator 9.

1.2.2. Internal Part

The internal part of the evaporator serves to deliver the stream ofneutral Cs onto the zone to be analysed in the form of a jet ofsufficiently reduced diameter to avoid any contamination of the analysischamber 3. The whole of the internal part is mounted on the main chamberof the CMS machine by means of a union 10 adjustable in distance andinclination to allow optimum positioning of the spot of Cs⁰ on theuseful zone. The axis of the evaporator forms an angle of 45° withrespect to the normal to the sample.

When the end piece of the external part guide tube is brought intocontact with the entry plate of the internal part by means of thetranslator 9, the Cs vapour can propagate itself in a second stainlesssteel tube 11 which has an internal diameter of 8 mm and is 177 mm long.At the end of this tube 11 is a metal support plate on which is mounteda motorised obturation system 12 which allows the stream of Cs⁰ to beadjusted continuously between 0 and 100%, using a disc with slit ofcontinuously variable width 16 driven by a stepper motor 17 (see FIG.6).

At the exit from the obturation system 12 is located the end piece 13 ofthe gun which serves to further reduce the diameter of the jet of Cs⁰.This end piece is formed by stainless steel cylinder 45 mm long with aninternal diameter of 5 mm which ends in a cone with an escape hole 2 mmin diameter.

All the pipes in the internal part can be heated by means of a secondheating wire 14. The temperature is monitored on the end piece of thegun by means of another chromel-alumel thermocouple 15.

1.3. Characterisation

1.3.1. Thermal Behaviour

Because of their different weights and environments, i.e. vacuum for theinternal part, atmosphere for the external part, the external andinternal parts present fairly different thermal behaviours. It is thusnecessary to apply a much greater power to the heating element of theexternal part than to that of the internal part when one wishes to raisethe two tubes to the same temperature. On the other hand, the fact thatthe external part loses a considerable proportion of the heat receivedto the external environment via the evaporation block and via thebellows of the translation system, the surface of which is comparable tothat of a radiator, allows the temperature of the reservoir to belowered quickly with a view to stopping the evaporator.

While the internal part must be raised rapidly to a temperature of 110°C. to avoid any risk of condensation and obturation in the varioustubes, the heating power brought to the external part should beincreased in successive steps to guarantee progressive heating of the Csreservoir.

1.3.2. Cs⁰ Evaporation Rates: Calibration of Deposition Rates

In order to be able to measure the stream of neutral Cs delivered by theevaporator, a quartz microbalance system was installed on the CMSmachine. The measuring device consists of a Leybold Inficon sensorequipped with a quartz crystal covered with a layer of gold and with aworking frequency of 6 MHz. Using a Leybold Inficon XTM/2 depositionmonitor and after a careful calibration taking into account the adhesionof the Cs film deposited on the quartz, we thus manage to determine thestream of Cs⁰ with an accuracy of 0.01 Å/s, which is equivalent to adeposition rate of approximately 4·10⁻³ monolayers per second.

The sensor is installed by means of two tubes on a translation systemoccupying a flange of the main chamber on the side diagonally oppositethe evaporator (not shown). This assembly enables the sensor to be movedon a horizontal axis located at the same distance from the extractionoptics as the sample during the analyses. Thus the sensor can be broughtin front of the stream of Cs⁰ at the point where the sample is normallypositioned and can be removed to be replaced by the sample-holder forthe analyses. In addition, the system set up enables a profile to beproduced through the jet of Cs⁰ by sweeping the diagonal of the beamwith the sensor.

FIGS. 7 and 8 represent the calibration curves enabling the stream of Csto be adjusted by adapting the heating power of the reservoir.Independently from the power P_(ext) applied, the internal part of theevaporator is raised during all the analyses to 110° C. From thesecalibration curves it can be concluded that Cs⁰ deposition rates on thesample of the desired order of 1 Å/s can be reached with reasonableheating powers and temperatures.

As curves 7 and 8 exhibit an exponential change in the deposition rateas a function of the heating power P_(ext) applied to the reservoir andof the temperature of the reservoir, even greater flow rates appearpossible by increasing the temperature by only a few degrees.

Finally, we should note that the heating and evaporator flow ratestabilisation phases result in a time of approximately 90 min. to reacha deposition rate of 1 Å/s after start-up of the evaporator.

1.3.3. Pressure Conditions

The reservoir temperature range (70 ° C. to 90 ° C.) required for theevaporator to output the necessary flow corresponds according to the Cssaturating pressure curve to a pressure range in the source of between1·10⁻⁴ mbar and 4·10⁻⁴ mbar. This value appears to be realistic in viewof the length (approximately 40 cm) and the small diameter (between 2 mmand 8 mm) of the pipe connecting the reservoir to the main chamberthrough which the pumping is carried out.

In addition, we observe that the pressure in the analysis chamberremains at a quite acceptable level during the operation of theevaporator (4·10⁻⁸ mbar to 1·10⁻⁷ mbar for deposition rates between 0.3Å/s and 3.5 Å/s).

1.3.4. Stability of Deposition Rates

By recording the deposition rates indicated by the microbalancecontroller for various heating powers and therefore for different valuesof the evaporator flow rate (see example in FIG. 9 for P_(ex)=85 W), wedetermined a deposition rate stability Δv_(D)/v_(D)=2% over 60 min.

1.3.5. Dimensions of Cs⁰ Beam

To evaluate the diameter of the spot of Cs⁰ on the sample, we measuredthe deposition rate with the quartz balance for various heating powers,while moving the quartz balance on its horizontal axis using thetranslation system (see example in FIG. 10).

A first method for judging the diameter of the spot of Cs⁰ producedconsists in approximating the profile of the beam with a Gaussian. Inthis case, we obtain an average curve width of 7.5 mm. By choosing thewidth at mid-height as a criterion, the diameter of the spot can beevaluated at 8.7 mm. Finally, the average width at the base of theprofile (v_(D)=0) is 18.5 mm.

Given the dimensions of the sample holder (10 cm×7.5 cm), we canconclude that the whole stream of Cs⁰ delivered by the evaporatorremains confined on this plate and that there is consequently no reasonto fear contamination of the analysis chamber.

In addition, the beam profiles formed made it possible to verify thatthe maximum Cs⁰ intensity lies directly below the extraction nose of thesecondary optics, which means that the spot is correctly centred on thezone to be analysed.

1.3.6. Purity of Cs⁰ Deposit

The cleanliness of the Cs deposit is a crucial point for the use of theCs⁰ evaporator during the analyses. A contamination of the Cs vapourwith impurities would lead to an increase in the detection limits forcertain elements given that the signal of the element in question wouldbe affected by a background noise of varying degrees of intensity.

An elementary analysis of the layer of Cs deposited was carried out bytwo different means. First, samples of Si and AsGa exposed to the streamof Cs⁰ were bombarded with Ga⁺ ions and it was thus possible to recordmass spectra in situ. Secondly, at the end of these experiments, thesample of Si used was taken out of the CMS machine to be studied on theLeica 430i Scanning Electron Microscope (LEO), given that the differencein mass between Cs (133 a.m.u.) and Si (28 a.m.u.) ought to guarantee agood contrast in electronic imaging. An EDX spectrum was notablyproduced in this second analysis.

The spectra obtained in SIMS analysis and in SEM analysis were comparedwith spectra of the same type produced beforehand on the untouched Siand AsGa samples. This comparison shows that the mass spectra producedwhile the evaporator was depositing Cs on the surface are composed ofthe same peaks as the spectra of the untouched samples plus the typicalpeaks (Cs⁺, Cs₂ ⁺, GaCs⁺ together with SiCs⁺ or AsCs⁺) due to thepresence of Cs atoms on the samples of Si or AsGa. However, there is noexplicit trace of any contaminant. We observe for example in thiscontext that the peaks of the alkaline elements Na and K which mightfeature among the impurities contained in the Cs bulb do not come out atsignificantly higher intensities.

As regards the EDX spectra, we can note that the spectrum recorded afterthe deposition comprises a fairly clear peak of O. This could beexplained by the fact that the Cs deposited on the sample reacted withthe ambient air during the transfer of the sample between the CMSinstrument and the SEM to form Cs oxides.

In the light of these results, we can conclude that the Cs deposition attypical rates around 1 Å/s does not lead to any detectable majorcontamination which might hinder the analyses carried out under standardGa⁺ bombardment conditions.

2. Experimental Procedure

2.1. Principle

2.1.1. Characteristic Parameter τ

The Cs concentration is adjusted by means of the ratio between thecurrent delivered by the analysis beam, which was for test purposes abeam of Ga⁺ ions, and the quantity of Cs deposited by the evaporator.

To characterise the analysis conditions, we define the parameter τexpressing the ratio between the stream of Cs⁰ and the Ga⁺ current bymeans of the deposition rate v_(D) and the erosion rate v_(er):

$\begin{matrix}{\tau = \frac{v_{er}}{v_{D}}} & (1)\end{matrix}$

Qualitatively, we can therefore now assert that the Cs concentration(c_(Cs)) and the parameter τ vary in opposite directions. The greater τbecomes, the more the quantity of sputtered matter increases and thelower the proportion of Cs becomes.

2.1.2. Cs Concentration Produced

Let us consider a sample of density ρ_(M). If we designate by Y thesputtering yield characterising the primary bombardment conditions forthe type of sample considered and by ρ_(Cs) the atomic density of thelayer of Cs formed, the Cs concentration can be written:

$\begin{matrix}{c_{Cs} = \frac{1}{1 + {\left( {\tau - 1} \right) \cdot \frac{\rho_{M}}{\rho_{Cs}}} + \frac{\tau}{Y}}} & (2)\end{matrix}$

It is important to note that, according to the relationship above, theCs concentration depends on the characteristics of the sampleanalysed—by way of its density ρ_(M) and its characteristic sputteringyield Y for the given bombardment conditions—together with the ratio τbetween the Cs⁰ erosion and deposition rates, but not the values inthemselves of these two rates.

2.2. Experimental Study

2.2.1. Experimental Conditions

To practically test the method of analysis consisting of a Ga⁺bombardment accompanied by a deposition of Cs⁰, we used samples ofaluminium, silicon and nickel, given that these materials cover aconsiderable range on the work function scale.

The reservoir of the Cs⁰ evaporator was heated with varying powers inorder to obtain different values of the deposition rate v_(D).

The Ga⁺ gun was operated at an energy of 28 keV and the sample polarisedat +4500 V was positioned at a distance d=2.5 mm from the extractionnose. These conditions result in an angle of incidence of the primaryions of è=54° with an impact energy of E=23.5 keV.

In order to be able to vary the parameter ô while at the same timekeeping the deposition rate constant, we changed the density ofbombardment with Ga⁺ ions by adapting the dimensions of the scanningsurface.

A summary of the corresponding experimental conditions are given inTable 1.

TABLE 1 Sample holder Distance d = 2.5 mm Sample voltage U = +4500 V Ga⁺gun Primary energy E′ = 28 keV Primary current 4.0 nA < I_(p) < 4.2 nAScanning surface 38 × 38 μm² to 190 × 190 μm² Cs⁰ evaporator Internalheating power P_(int) = 12 W Temperature of internal part T_(int) = 110°C. External heating power 80 W < P_(ext) < 105 W Temperature of externalpart 75° C. < T_(int) < 87° C. Deposition rate 0.9 Å/s < v_(D) < 3.5 Å

2.2.2. Accessible Cs Concentration Range

FIG. 11 shows, in an example, the change in the Cs concentrationdetermined experimentally—knowing the quantity of Cs deposited, thenumber of Ga⁺ ions incorporated in the volume analysed and the volumesputtered—over the given erosion rate range for four different Cs⁰deposition rates for the aluminium sample.

To enable a comparison between these experimental values of c_(Cs) andthe theoretical evolution established in the relationship, we calculatedfor each measurement point the ratio τ between the erosion rate and thedeposition rate and plotted all the points thus obtained on the samegraph.

The experimental curve of the aluminium sample resulting from thistransformation operation is plotted together with the respectivetheoretical behaviour of c_(Cs) as a function of the parameter τ in FIG.12.

Several conclusions can be drawn from FIG. 12. First of all, the resultsupplied by the theoretical relationship, namely that the value ofc_(Cs) depends only on the ratio between the erosion and depositionrates and not on the individual values, was corroborated experimentally.The four curves obtained for each sample when converting the erosionrate scale into a parameter τ scale are almost perfectly superimposed.

Secondly, the graphs show a very good agreement between the experimentalresults and the theoretical forecasts for values of τ greater than about6. For lower values, the gap between the two curves becomes increasinglywider, while their general appearance (shape of the curve) remainsgenerally the same. We find the same limitations concerning the area ofvalidity of the theoretical equation (2) encountered at the time ofestablishment of this relationship: the theoretical forecasts arecorrect provided that the Cs concentration is not too great, i.e. τ doesnot become too small.

2.2.3. Study of Useful Yields Attained

A systematic study clearly revealed that the useful yield—whichexpresses the sensitivity of the analysis—of the Cs_(x) ⁺ and MCs_(x) ⁺clusters detected by sputtering the sample with an ion beam whilesimultaneously depositing neutral Cs depends solely on the ratio τbetween the erosion rate and the deposition rate, but not on the valuesof these two rates as such (see the example in FIG. 13). we weretherefore able to verify experimentally the following transitivity rule:given that the useful yield is a function of the Cs concentration, andas this latter value depends on the parameter τ, the useful yield of theCs_(x) ⁺ and MCs_(x) ⁺ clusters is a function of τ.

2.3. Comparison Between the Performances of the CMS Instrument and Thoseof the Cameca IMS 4f and IMS LAM

It is interesting to summarise in the same comparative table the maximumvalues of the useful yields determined above for the method ofsputtering by an ion beam using a simultaneous deposit of Cs⁰ and thoseattained for the same three samples on the Cameca IMS 4f and Cameca IMSLAM instruments operated under standard conditions, namely an extractionvoltage of +4500 V and a primary energy of 10 keV (Table 2).

These three instruments use the same secondary optics and measurementshave shown that the transmission of these secondary optics areidentical.

TABLE 2 Sample Signal CMS Cs⁰ IMS 4f/IMS LAM Al (Φ = 4.28 eV) Cs⁺ 3.6 ·10⁻¹ 3.8 · 10⁻³ AlCs⁺ 1.3 · 10⁻⁴ 5.4 · 10⁻⁶ Si (Φ = 4.85 eV) Cs⁺ 3.6 ·10⁻¹ 6.5 · 10⁻² SiCs⁺ 1.6 · 10⁻⁵ 1.1 · 10⁻⁵ Ni (Φ = 5.15 eV) Cs⁺ 3.6 ·10⁻¹ 3.1 · 10⁻¹ NiCs⁺ 8.5 · 10⁻⁵ 2.0 · 10⁻⁴

It can be seen that the useful yields determined using a deposit of Cs⁰are higher (for materials with a low work function) and only slightlylower (for materials with a high work function) than the correspondingvalues obtained on conventional Cameca instruments. These differencescan be explained by considering factors such as the work functionvariations induced by the Cs deposit, the reductions resulting from theprobability of ionisation of the Cs, the cluster formation processes,the spatial and temporal correlation of the partners involved, etc.

Considering this comparison concerning the useful yields obtainedtogether with the successful separation of the Cs sputtering andimplantation stages during the analyses in MCs_(x) ⁺ mode, the potentialof the Cs⁰ deposit is undeniable.

2.4. Automation

Continuous recording of the secondary signals makes it possible to checkat any moment that the analysis conditions are optimised, and ifnecessary, for example on passage from one layer to another, to adaptthe stream of neutral Cs via the obturation system. The principle ofthis continuous and automatic optimisation is as follows.

By comparing the values of the various intensities at the moments t andt+dt, we define the value of ΔI as follows:

${\Delta\; I\mspace{14mu}{comparator}\text{:}\mspace{14mu}\Delta\; I} = {\begin{Bmatrix}0 \\1\end{Bmatrix}\mspace{14mu}{if}\mspace{14mu}{\begin{Bmatrix}{{I\mspace{11mu}\left( {t + {dt}} \right)} < {I\mspace{11mu}(t)}} \\{{I\mspace{11mu}\left( {t + {dt}} \right)} > {I\mspace{11mu}(t)}}\end{Bmatrix}.}}$

In addition, it is possible to define the opening (OPEN) of theobturator together with its variation (ΔOPEN). The value of C(“comparator”) is then given by carrying out the logical operationC=ΔOPEN ⊙ ΔI, taking the initial condition ΔOPEN=0. Table 3 shows thecorresponding logic table.

TABLE 3 Δ OPEN ΔI C 0 0 1 0 1 0 1 0 0 1 1 1The automation programme procedure is eventually as follows:

${❘\begin{matrix}{{\Delta\mspace{14mu}{OPEN}} = 0} \\\begin{matrix}{{loop}\text{:}} & {{calculate}{\;\mspace{11mu}}\Delta\; I} \\\; & {C = {\Delta\mspace{14mu}{{OPEN} \odot \Delta}\; I}} \\\; & {{\Delta\mspace{14mu}{OPEN}} = C}\end{matrix}\end{matrix}}\quad$

The electronics of the obturator is directly coupled to the automationsystem and reacts to its orders: if ΔOPEN=1, the opening of the shutteris increased; if ΔOPEN=0, the opening is decreased.

2.5. Conclusions

We have shown that the analysis technique consisting of a primarybombardment with Ga⁺ ions accompanied by a simultaneous deposit of Cs⁰does indeed allow the Cs concentration to be varied continuously overthe whole range. The value of this range depends only on thecharacteristics of the material analysed and the relationship betweenthe erosion and deposition rates, but not on the individual values ofthese two rates.

Our experiments have proved that it is possible to carry out analyses inMCs_(x) ⁺ mode by sputtering the sample with a Ga⁺ ion beam whilesimultaneously depositing neutral Cs with very promising useful yields.We have highlighted the fact that the behaviour of the Cs_(x) ⁺ andMCs_(x) ⁺ signals detected depends solely on the ratio τ between theerosion rate and the deposition rate, and not on the values inthemselves of these two speeds.

This becomes very important within the perspective of a low-energyprimary bombardment with a view to an improvement of the depthresolution: in this case the sputtering yield and consequently theerosion rate take very low values; by then lowering the deposition rateto attain optimum values of the parameter τ, we succeed in optimisingthe secondary signals in which we are interested.

3. EXEMPLES OF APPLICATION

In this section, we shall present a few examples of applications of thisanalysis technique. For these purposes, we shall make a distinctionbetween the two main types of application of the SIMS technique, namelythe depth profiles and ion imaging.

3.1. Depth Profiles

3.1.1. Implant of Mg and In in Si

The sample considered consists of a silicon substrate in which magnesiumand indium ions have been implanted at an energy of 300 kev and at adose of 10¹⁵ atoms/cm².

As the elements involved come out well in the form of MCs⁺ clusters, adepth profile of this sample was made by recording the MgCs⁺, InCs⁺ andSiCs⁺ signals. In addition, the Cs⁺ and Ga⁺ secondary intensities weremeasured in order to get an indication of the stability of theexperimental conditions. Table 4 shows a summary of the correspondingexperimental conditions

TABLE 4 Ga⁺ gun Primary energy E′ = 28 keV Primary current I_(p) = 4.2nA Scanning surface S = 70 × 70 μm² Cs⁰ evaporator Internal heatingpower P_(int) = 12 W Temperature of internal part T_(int) = 110° C.External heating power P_(ext) = 90 W Temperature of external partT_(int) = 80° C. Deposition rate v_(D) = 1.5 Å/s Secondary opticsSample-extraction distance d = 2.5 mm Sample voltage U = +4500 VDiameter of zone analysed ψ = 42 μm Mass resolution M/ΔM = 300 Energybandwidth ΔE = 130 eV

In accordance with the results established in the course of our study,we chose a primary bombardment density and a Cs⁰ deposition rate suchthat the parameter τ is close to the optimum value (τ=4.5) for theformation of the MCs⁺ clusters from a Si matrix. Indeed, the chosenexperimental conditions lead to an erosion rate of 6.5 Å/s, and wetherefore arrive at a value τ=4.3. In practice, this value of τ isadjusted by varying the dimensions of the primary raster for a givenprimary current intensity and a given deposition rate, in order toposition oneself at the critical threshold common to the Cs⁺ and MCs⁺secondary intensities.

FIG. 14 shows that our analysis technique makes it possible to obtainvery good quality implantation profiles. Because of their lower mass atthe same implantation energy, the Mg ions penetrated more deeply intothe sample. It should also be noted that the matrix signals linked tothe primary elements remain perfectly stable during the analysis.

3.1.2. Implant of Ti and Cu in Al

The second example consists of a sample of aluminium in which titaniumand copper ions were implanted at an energy of 180 keV and at a dose of10¹⁶ atoms/cm².

Once again, all the interesting elements can be detected in the form ofMCs⁺ clusters and therefore the TiCs⁺, CuCs⁺ and AlCs⁺ signals weremeasured together with Cs⁺ and Ga⁺ as a function of time.

As a result of its low work function, the aluminium matrix reacts muchmore critically to the Cs deposit than is the case for Si. Consequently,and in accordance with the results established during our experimentalstudy of the formation of the MCs_(x) ⁺ clusters, we set the parameter τto a higher value.

By adapting the dimensions of the primary raster to position ourselvesat the critical threshold common to the Cs⁺ and MCs⁺ secondaryintensities, we arrive at a value of τ=5.6, which is in full agreementwith the value determined as being optimal (τ=5.9) for the analysis ofthe MCs⁺ clusters from an Al matrix. Table 5 shows a summary of theexperimental conditions.

TABLE 5 Ga⁺ gun Primary energy E′ = 28 keV Primary current I_(p) = 4.2nA Scanning surface S = 65 × 65 ìm² Cs⁰ evaporator Internal heatingpower P_(int) = 12 W Temperature of internal part T_(int) = 110° C.External heating power P_(ext) = 90 W Temperature of external partT_(int) = 80° C. Deposition rate v_(D) = 1.5 Å/s Secondary opticsSample-extraction distance d = 2.5 mm Sample voltage U = +4500 VDiameter of zone analysed Ψ = 42 ìm Mass resolution M/ΔM = 300 Energybandwidth ΔE = 130 eV

3.1.3. Implant of F and Al in InP

While we exclusively measured MCs⁺ type signals in the two first depthprofiles, we chose for this third example a sample of InP in which weimplanted both an electropositive element, in this case Al, which isanalysed in the form of MCs⁺ clusters and an electronegative element,namely F, which is best detected in the form of MCs₂ ⁺ clusters. The Alimplantation energy was 180 keV while that of the F was reduced to 130keV because of its lower atomic mass. The implanted doses were set at10¹⁶ atoms/cm² for the two elements.

For this depth profile, it is therefore required to record MCs⁺ and MCs₂⁺ type ions together with the Cs⁺, Cs₂ ⁺ and Ga⁺ secondary signals whichserve as indicators of the stability of the analysis conditions. Takinginto account that the optimum values of τ depend on the particular typeof secondary ion considered (Cs⁺, Cs₂ ⁺, MCs⁺, MCs₂ ⁺), we cannotsimultaneously optimise all the signals and we are therefore obliged tocompromise concerning the setting of the parameter τ. Afterexperimentally determining the optimum τ values for the MCs⁺ ions(AlCs⁺, InCs⁺, PCs⁺) and for the FCs₂ ⁺ clusters by way of variations ofthe dimensions of the primary raster, we set the final value of τ forour analysis at an intermediate value (τ=7.5) leading to major secondaryintensities for all the signals under interest. Table 6 shows a summaryof the corresponding experimental conditions.

TABLE 6 Ga⁺ gun Primary energy E′ = 28 keV Primary current I_(p) = 4.2nA Scanning surface S = 112 × 112 μm² Cs⁰ evaporator Internal heatingpower P_(int) = 12 W Temperature of internal part T_(int) = 110° C.External heating power P_(ext) = 90 W Temperature of external partT_(int) = 80° C. Deposition rate v_(D) = 1.5 Å/s Secondary opticsSample-extraction distance d = 2.5 mm Sample voltage U = +4500 VDiameter of zone analysed Ψ = 23 m Mass resolution M/ΔM = 300 Energybandwidth ΔE = 130 eV

We observe in FIG. 16 that all the matrix signals (PCs⁺ and InCs⁺)together with those linked to the primary elements (Cs⁺, Cs₂ ⁺ and Ga⁺)take approximately 90 seconds to stabilise. This initial instabilityshould indicate the existence of a quite considerable transient statefor this type of material, which is sputtered very easily.

Concerning the two elements implanted, we observe again that theimplantation profiles recorded are of very good quality.

3.2. Ion Imaging

To demonstrate the possibility of producing ion images in MCs_(x) ⁺ modeby scanning the surface of the sample with a fine Ga⁺ ion beam whilesimultaneous depositing Cs⁰ atoms on it, we recorded images of a grid bysuccessively detecting four different elements (FIG. 17).

The chosen grid consists of a substrate of aluminium incorporatingcopper bars 5 μm wide at 20 μm intervals.

Given that the enlargement of the ion image directly depends on thedimensions of the primary sweep and that strong secondary intensitiesare not required, the parameter τ is not optimised in this example.Table 7 shows a summary of the corresponding experimental conditions

TABLE 7 Ga⁺ gun Primary energy E′ = 28 keV Primary current I_(p) = 600pA Scanning surface S = 38 × 38 μm² Cs⁰ evaporator Internal heatingpower P_(int) = 12 W Temperature of internal part T_(int) = 110° C.External heating power P_(ext) = 75 W Temperature of external partT_(int) = 75° C. Deposition rate v_(D) = 0.6 Å/s Secondary opticsSample-extraction distance d = 2.5 mm Sample voltage U = +4500 V Massresolution M/ΔM = 300 Energy bandwidth ΔE ≈ 5 eV

By detecting the CuCs⁺ and AlCs⁺ secondary ions, we manage to displayonly the bars forming the grid and the areas bounded by these bars.However, if we record the Cs⁺ or Cs₂ ⁺ signals, we obtain a contrastimage in which the areas between the bars appear to be raised.

1. Method for modifying electronic properties of a sample surface toanalytical ends, comprising in situ deposition of neutral cesium (Cs⁰),under ultra-high vacuum, said neutral cesium being enabled in the formof a collimated adjustable stream, said Cs⁰ deposition beingsimultaneously accompanied by a primary bombardment of said surface, inthe form of at least a beam comprising electrons and/or ions or neutralatoms or groups of atoms, or by an X-ray irradiation, intended to inducea secondary emission or sputtering of particles for analysis, out of thesurface, said sputtering comprising secondary electrons and/or Cs_(x)^(n+) and/or MCs_(x) ^(n+) positive clusters (x=1, 2) and/or M^(n−)negative ions and/or M^(m+) positive ions, M being a constituent of thesample material made of an atom or a group of atoms (n, m integers), theCs⁰ deposition being decoupled from the primary bombardment conditions,to provide a simultaneous optimization of deposited Cs⁰ concentrationand analytical characteristics, such as the depth resolution,characterized in that said optimized deposited Cs⁰ concentration ischosen only by adjusting the ratio (τ=V_(er)/V_(D)) between the erosionrate (v_(er)) and the Cs⁰ deposition rate (V_(D)), for a given sampleand given primary bombardment conditions.
 2. Method according to claim1, wherein said optimized deposited Cs⁰ concentration is continuouslyadjustable according to the relation:${c_{Cs} = \frac{1}{1 + {\left( {\tau - 1} \right) \cdot \frac{\rho_{M}}{\rho_{Cs}}} + \frac{\tau}{Y}}},$wherein ρ_(M) is the density of the sample constituent, ρ_(Cs) theatomic density of the layer of Cs formed and Y the sputtering yieldcharacterizing the primary bombardment conditions for the sampleconsidered.
 3. Method according to claim 1, wherein the stream of Cs⁰ isprovided and collimated in a column by means of: a temperatureadjustment of an evaporator comprising a metallic cesium reservoir,and/or an aperture control of a motorized obturator located in the pathof the cesium stream.
 4. Method according to claim 3, wherein thereservoir temperature range is maintained between 70 and 90° C.,corresponding to a pressure range from 1.10⁻⁴ to 4.10⁻⁴ mbar and in thatthe stability of the deposition rate is about 2% over 60 minutes. 5.Method according to claim 3, wherein the stream of Cs⁰ is automaticallyand continuously adapted via the obturator.
 6. Method according to claim1, wherein it is coupled to static or dynamic Secondary Ion MassSpectroscopy (SIMS), preferably operating in the MCs_(x) ⁺ mode (x=1,2).
 7. Method according to claim 6, wherein the deposition rate of Cs⁰is continuously adjustable in the range from 0 to 10 Å/s, correspondingabout to 0–4 monolayers per second.
 8. Method according to claim 1,wherein it is coupled to electron spectroscopy.
 9. Method according toclaim 8, wherein electron spectroscopy is selected from the groupconsisting of Auger Electron Spectroscopy (AES), Electron Energy LossSpectroscopy (EELS), X-Ray Photoemission Spectroscopy (XPS) andUltraviolet Photoemission Spectroscopy (UPS).
 10. Method according toclaim 1, wherein it further enables a stream of a chemical element otherthan Cs, evaporated under ultra-high vacuum, to create secondaryemission for analytical purposes of M₁M₂ ^(n+) clusters or M₂ ^(m−) ionsor M₂ ^(m+) ions (n, m integers) or electrons, wherein M₁ and M₂ arerespectively the atoms or groups of atoms constituted by the chemicalelement other than Cs and the atoms or groups of atoms from the sample.11. Method according to claim 1, wherein the sole adjustable depositionrate of Cs⁰ or a chemical element other than Cs to an optimized valueenables to optimize the intensity of secondary particles emitted by thesample.
 12. Method according to claim 1, wherein the useful yield, i.e.the sensitivity of the secondary emission species, preferably M^(n−),M^(m+), and still more preferably Cs_(x) ^(n+) and MCs_(x) ^(n+), isapproximately optimized solely by adjusting said ratio (τ).
 13. Energyand/or mass analyzer instrument for carrying out the method according toclaim 1, comprising a neutral cesium (Cs⁰) deposition column, capable ofdelivering an adjustable and stable stream of pure neutral cesium, saidneutral cesium column being usable simultaneously with a primarybombardment or a primary irradiation column, and comprising anevaporation block (1) including a reservoir (5) filled with puremetallic cesium, equipped with temperature control means (6,7),prolongated by a tube (2,11) up to a gun end piece (13) located close tothe sample and equipped with beam collimation means (12), characterizedin that said tube (2, 11) and gun end piece (13) equipped with beamcollimation means (12) are further equipped with temperature controlmeans (8,14,15) for preventing condensation and obturation risks. 14.Instrument according to claim 13, wherein the evaporation block (1) islocated in an external part which can be isolated from the main chamber(3) of the instrument by means of a gate valve (4) and capable of beingseparately pumped and vented.
 15. Instrument according to claim 13,wherein said beam collimation means comprise a motorized continuouslyadjustable obturator (12), preferably comprising a rotary disk using aslit of continuously variable width (16), said disk being driven by astepper motor (17).
 16. Instrument according to claim 13, wherein, atthe operation temperature, the neutral cesium (Cs⁰) is in liquid stateand the evaporation block (1) lies with an inclination angle such assaid liquid remains in the bottom of the reservoir (5) under gravityeffect.
 17. Instrument according to claim 13, preferably a static ordynamic secondary ion mass spectrometry (SIMS) instrument, comprising aprimary bombardment column and a secondary column equipped withsecondary ion extraction means, a mass spectrometer, preferably of thetype TOF (Time-Of-Flight), quadrupolar or with magnetic sector and iondetection means.